Wireless electronic communications encounter particular difficulties in certain types of environments or situations. In urban environments, reflections and multi-path are problematic. In underwater or underground environments, signal attenuation presents a particular problem for RF signals. In military applications, signal interception and signal jamming are significant concerns with RF communications.
Accordingly, wireless communications systems have been developed that rely upon magneto-inductive technology. Magneto-inductive communications use quasi-static low frequency AC magnetic fields. A quasi-static magnetic field differs from an electromagnetic field in that the electric field component is negligibly small. A quasi-static magnetic field does not propagate as an electromagnetic wave, but instead arises through induction. Accordingly, a quasi-static magnetic field is not subject to the same problems of reflection, refraction or scattering that radio frequency electromagnetic waves suffer from, and may thus communicate through various media (e.g. earth, air, water, ice, etc.) or medium boundaries. It is also very difficult to intercept or eavesdrop on magneto-inductive communications since interception would require an antenna properly tuned to the specific magnetic field.
Typical magneto-inductive (MI) systems include a magneto-inductive transmitter and a magneto-inductive receiver, and operate in the range of a few hundred Hz to 10 kHz. More typically, the operating frequency of an MI system is in the range of 500 to 3000 Hz. The MI transmitter and the MI receiver each have a coil antenna. In some cases, the antenna may be single loop of wire. In others, the antenna may be a helical coil of wire with multiple turns. Some MI systems may be capable of two-way communication and, thus, may feature MI transceivers. The MI transceiver may use a single antenna for both transmission and reception; although it may be advantageous in some instances to have a different loop length for transmission and reception. Accordingly, in some instances, the MI transceiver may have two separate antennas or may have a single switchable antenna that is capable of altering its length depending on whether it is used in transmit or receive mode. An example of a switchable antenna is described in U.S. Pat. No. 6,333,723, entitled Switchable Transceiver Antenna, and owned in common herewith.
MI systems find application in undersea operations, mining, military, and other such fields. For example, MI systems may be used for wireless communications purposes, including, in some cases, the transmission of data communications or the transmission of audio for voice communications.
The robustness of MI communications and the resistance of the signal to interference, reflection, refraction, and other environmental attenuations make them particularly attractive for enabling communications in the mining industry, in emergency services, in military applications, and similar hazardous environments.
A difficulty arises in providing for an MI system that is easily portable by personnel. Emergency service personnel, military personnel, and the like, are already burdened with heavy equipment, so it would be advantageous to minimize the bulk and cumbersomeness associated with carrying a portable MI transceiver.